A data communications network generally includes a group of interconnected communication channels which provides intercommunication among a combination of elements or devices, for instance, computers, peripherals, etc. Historically, networks have been constructed by utilizing communication channels formed from coaxial cables and/or twisted pair cable configurations and interconnected via a suitable interface, or switching module.
Fiber optic cables are increasingly being used in the network industry, instead of coaxial cables and twisted pairs, because of their much broader bandwidth, better propagation properties, and other optimal transmission characteristics. Recently, the Fibre Channel protocol was developed and adopted as the American National Standard For Information Systems (ANSI). The Fibre Channel industry standard is described in detail in, for example, Fibre Channel Physical And Signalling Interface, Rev. 4.2, American National Standard For Information Systems (ANSI) (1993). The Fibre Channel industry standard provides for much higher performance and greater flexibility than previous industry standards by allowing for variable-length data frames, or packets, to be communicated through fiber optic networks which comply with the standard.
A variable-length frame 11 is illustrated in FIG. 1. The variable-length frame 11 comprises a 4-byte start-of-frame (SOF) indicator 12, which is a particular binary sequence indicative of the beginning of the frame 11. The SOF indicator 12 is followed by a 24-byte header 14, which generally specifies, among other things, the frame source and destination address as well as whether the frame 11 is either control information or actual data. The header 14 is followed by variable-length data 16. The length of the data 16 is 0 to 2112 bytes. The data 16 is followed successively by a 4-byte CRC (cyclical redundancy check) code 17 for error detection, and by a 4 byte end-of-frame (EOF) indicator 18. The frame 11 of FIG. 1 is much more flexible than a fixed frame and provides for higher performance by accommodating the specific needs of specific applications.
The Fibre Channel industry standard also provides for several different types of data transfers. A class 1 transfer requires circuit switching, i.e., a reserved data path through the network switch, and generally involves the transfer of more than one data frame, oftentimes numerous data frames, between the network elements. In contrast, a class 2 transfer requires allocation of a path through the network switch for each transfer of a single frame from one network element to another.
To date, fiber optic switches for implementing networks in accordance with the Fibre Channel industry standard are in a state of infancy. One such fiber optic switch known in the industry is ANCOR, which is manufactured by and made commercially available from IBM, U.S.A. However, the performance of the ANCOR switch is less than optimal for many applications and can be improved significantly. Moreover, the ANCOR switch is inflexible in that it provides for primarily circuit switching for class 1 transfers and is very limited with respect to frame switching for class 2 transfers.
Furthermore, most prior art fiber optic switches, if not all, allocate their ports on an as-available basis. There is generally no mechanism to insure that transfer requests through the switch are ultimately serviced by the switch. These switches rely heavily on probabilities, that is, that it is probable that a transfer request cannot be consistently denied access to a port for a lengthy period of time. However, in these prior art switches, some data frames are consistently refused access to a desired port. Moreover, this dilemma is worsened by resonances, or repetitive patterns, in the protocol of port accesses. That is, long sequences may result in a very regular periodic traffic pattern to one or more ports. If this pattern aligns with how port resources are allocated, the sequence could even end up unduly appropriating port resources and slow down other sequences.
Thus, a heretofore unaddressed need exists in the industry for new and improved systems for implementing the Fibre Channel industry standard for fiber optic networks with much higher performance than presently existing systems. Specifically, there is a significant need for a path allocation system and method for a fiber optic switch which can provide for both circuit switching and frame switching with high performance, high flexibility for a variety of applications, and insurance that transfer requests will ultimately be accommodated by the switch without reliance on probabilities of port availability.